The present invention relates to a system for controlling an electro-magnetic clutch for an internal combustion engine mounted on a car.
An electro-magnetic powder clutch is a known type electro-magnetic clutch. The electro-magnetic powder clutch comprises an annular drive member secured to the crankshaft of an engine, a magnetizing coil provided in the drive member, a driven member secured to the input shaft of a transmission spaced from and leaving a small gap from the drive member, magnetic powder provided therein, and a shift lever for changing the gears in the transmission. The shift lever is provided with a switch for the magnetizing coil, which is actuated by operating the shift lever. When the shift lever is shifted to the gear engaging position, the switch is closed so that electric current flows through the magnetizing coil to magnetize the drive member. As the accelerator pedal is depressed, the current applied to the coil increases. The magnetic powder is aggregated in the gap between the drive member and the driven member, so that the driven member is coupled to the drive member. The clutch current passing through the magnetizing coil is progressively increased according to the degree of depression of the accelerator pedal, while the clutch slips between the drive member and the driven member until the clutch current reaches the rated current. Thus, the car may be started by depressing the accelerator pedal without operating clutch pedal.
In order to smoothly start the car, it is necessary to detect the speed of the car and to control the clutch current according to the speed. In a system to meet such a requirement, a speed sensor is provided to generate a signal when the car speed exceeds a predetermined level. The system should be designed such that the clutch slips during low speed below the predetermined speed under the accelerating condition of the engine and the clutch perfectly engages when the car speed reaches the predetermined speed. Generally, the speed sensor is provided in the speedometer. In the system employing such a speed sensor, there is the disadvantage that when the car is decelerated at a speed below the predetermined speed, the braking action of the engine does not occur because of the disengagement of the clutch. Further, if the speedometer cable is broken, the speed sensor does not work. Therefore, if the speed sensor does not work in a deceleration condition, the circuit of the clutch coil is interrupted so that the clutch disengages. As a result, the braking action of the engine does not occur.